The present invention relates to analytic processing. Motion artifacts are reduced for coherent image formation.
For analytic processes in ultrasound imaging, predetected or complex data representing different lines in response to a same or different transmission and/or representing a same line in response to different transmissions is combined. For example, line synthesis is performed where complex data is combined in a filtering operation. As another example, analytic lines are interpolated from two adjacent lines of received complex data. Lateral whitening or other line-to-line filtering on predetected or analytic data may be performed.
Where combined data is responsive to transmissions separated by a delay, such as a delay associated with acquiring one or more Doppler samples, the phase of the complex data between two lines or acquisitions may be undesirably shifted due to tissue motion. If enough motion occurs between correlated lines, a motion artifact may be generated due to the analytic processing. For example, tissue motion at thirteen cm per second results in about 130 micrometers of motion in one millisecond. In a two-tap analytic filter, such as for line synthesis or interpolation at 2.5 MHz imaging frequency, the tissue motion corresponds to a 180° error. Two, three, or other numbers of taps for filtering analytic data may result in some cancellation due to tissue motion. Tissue motion of this speed is typical of the mitral valve. When the complex data is combined, the 180° phase shift due to the tissue motion cancels out the signal, resulting in a motion artifact. Small amounts of motion may produce varying amounts of cancellation. FIG. 1 shows a relationship of phase error to attenuation for one example of a two-tap synthesis or interpretation analytic process. A zero degree phase error shows zero dB or no drop out. A 180° phase error produces complete cancellation.
Motion resulting in multiple wave length changes between acquisitions of data to be combined may produce completely incoherent information in both amplitude and phase. Motion artifacts are generated as a function of both the delay between acquisitions of complex data to be combined and the amount of motion. Motion artifacts are generated even for short delays where the tissue is subjected to rapid motion. Very little motion may still create motion artifacts where long delays are provided between acquisitions.
Analytic line motion artifacts are reduced by video filtering the resulting detected data. Spatial, such as azimuth, smoothing reduces line artifacts. Since line artifacts are more likely apparent on edges of images or at steering angles away from normal to the transducer, any video filtering may vary on a line-by-line or beam-by-beam basis to remove expected artifact signals. However, spatial filtering or video filtering may reduce spatial resolution.
One common source of analytic line motion artifact is the delay provided between acquisitions of B-mode information for acquiring Doppler information. Where line-by-line of group of line interleaving is provided between B-mode and Doppler information, the delay to acquire Doppler information may result in motion artifacts in the B-mode images. To counteract the motion artifacts, one or more scan lines are reprimed. For example, data is acquired along a first scan line for B-mode imaging. Subsequently, data is acquired for Doppler imaging. Where data along the first scan line is going to be combined analytically with data along a second scan line acquired after the Doppler pulses, the data along the first scan line is reacquired to avoid a large delay and resulting motion artifact. Where analytic data representing three or more scan lines is combined, additional firings for repriming or reacquiring data may be used. As a result, the frame rate for B-mode imaging is decreased, resulting in lesser temporal resolution.